When drilling a hydrocarbon well, the process of insertion of a drill string is required for increasing the well drilling depth.
During insertion of a new string, continuous circulation of drilling mud must be ensured throughout the process until a complete pipe is obtained and the whole hydraulic circuit is restored. Indeed, pressure drops or variations in mud circulation have been found to cause considerable structural stresses in the well being drilled, which involve collapse in encased structures of the well being drilled.
In order to ensure such continuous circulation of drilling mud throughout the drilling process, and hence also during the steps of insertion of new drill strings or removal of existing strings, devices have been long provided, for ensuring steady circulation of drilling mud even during insertion or removal of a drill string.
A device of the aforementioned type is disclosed in the prior art document U.S. Pat. No. 3,298,385. Particularly, this prior art document shows the possibility of using a flapper valve having a single shut-off member for selectively regulating the passage of the drilling mud through the central axial conduit of the valve or, alternatively, through a lateral passage interposed between opposite ends of the valve.
The technical solution of using a single shut-off member to provide the aforementioned selective regulation of the passage of the drilling mud through the central axial conduit of the valve or, alternatively, through the lateral passage, is preferred over the use of two distinct shut-off members, as disclosed, for instance, in the prior art document U.S. Pat. No. 7,845,433, amongst other things due to the fact that there is no uncertainty about the closing position assumed by the single shut-off member.
Nevertheless, it shall be noted that, while the use of a single shut-off member to alternatively close either the central axial conduct of the valve or the aforementioned lateral passage interposed between the opposite ends of the valve provides advantages in terms of operation, it poses serious problems concerning proper positioning and centering of the body of the shut-off member within the central axial conduit of the valve. This is because the single shut-off member must be able to ensure a tight sealing effect, both in a first angular position, with the seat of the shut-off member oriented transverse to the axis of the valve, and in a second angular position, with the seat of the shut-off member longitudinally extending along the axis of the valve. Therefore, even a minor angular or axial positioning error with respect to the valve body may cause an imperfect sealing action of the shut-off member in at least one of the two seats of the shut-off member, which is not compatible with the high pressures of the drilling mud.
Particularly proper axial positioning of the body of the shut-off member and the point at which it is hinged to the valve body is problematic. In an attempt to tackle this problem, the valve body and the parts that form the moving shut-off assembly should be formed with very strict processing tolerances but sometimes this has not been practically found to be sufficient. During assembly of the valve and the moving shut-off member therein, the processing tolerances of the various parts may sum up and sometimes cause imperfect closure of the shut-off member in both seats of the shut-off member.
For the aforementioned reasons, these valves comprising a single shut-off member have not gained success in this field heretofore.
Therefore, there is an unfulfilled need of ensuring proper closing operation of the shut-off member in both closing positions, irrespective of any particular critical condition that may be encountered during assembly due to the summation of the tolerances of the various connections that have been made.